1. Field of the Invention
The present invention relates to a memory interface, and more particularly, to a memory access control apparatus capable of writing image data in an external memory or reading the image data.
2. Description of the Conventional Art
Various image display apparatuses which are recently being developed need to non-linearly format-convert image data, that is not only in a horizontal direction but also in a vertical direction. As an application of the format-conversion, there are a pincushion method, a keystone method, and etc., which can be applied to various display apparatuses.
For example, in an image display apparatus such as a projector for projecting an image on a screen, when an image is displayed on a screen, a phenomenon that an image displayed on the screen is non-linearly distorted by the screen or by an optical mechanism limit is generated. Also, in an image display apparatus based on a general brown tube, an image distortion is not generated at the center of the brown tube by a screen of a curved surface but an image distortion is generated in an edge direction of the brown tube. According to this, the image display apparatus reverse-converts the distorted image into the original image in an additional format conversion process block for compensating a partial distortion of the image. According to this, an actual image displayed on the screen is normally realized without a distortion, so that the user can see an image in an optimum state.
The image display apparatus outputs image data non-linearly when an image is to be displayed on the screen thus to display the original image on the screen without distortion. Functions for non-linearly processing image data include a tilt function, a pincushion function, a keystone function, etc., which are also known as a warping function. In order to implement the warping function, the image data has to be accessed to an external memory in a vertical direction or a horizontal direction.
However, in the conventional memory access control apparatus, the warping function is performed by storing the image data in an external memory in a horizontal direction by a raster scan method and then reading the stored image data in a horizontal direction. Therefore, in the conventional memory access control apparatus, a memory access latency becomes very great thus not to be able to smoothly read image data from the external memory, thereby lowering the stability of the entire system. Hereinafter, a process for storing image data in the external memory in accordance with the conventional art will be explained with reference to FIG. 1.
FIG. 1 is a view for explaining a storage principle of image data in a memory access control apparatus in accordance with the conventional art.
As shown, the conventional memory access control apparatus stores inputted image data by sequentially increasing a column address inside the external memory. That is, since the conventional memory access control apparatus consecutively processes the image data as an image line unit, a bank change of the external memory is not generated when the image data stored in the external memory is to be accessed in a horizontal direction of an image frame. According to this, the image data can be processed with a minimum memory access latency.
In the conventional memory access control apparatus, since a minimum memory access latency is used when adjacent image data stored in the external memory are accessed, a function of the entire system can be enhanced. Herein, the memory access latency is a delay time generated when image data stored in different banks and rows of the external memory are accessed, and can be represented as the number of memory operation clocks.
However, in the conventional memory access control apparatus, a considerable memory access latency is generated when image data sequentially stored in the external memory is read in a vertical direction of the image frame.
For example, when an interlaced scanning image of 512 k×64-bit×4 banks SDRAM is accessed, 8 pixel data, that is, 8 bytes are stored in one column in the conventional memory access control apparatus. Therefore, in order to store one image line inside one image frame in the SDRAM, 240 (1920/8) columns are necessary. Generally, the Nth bank of the Nth row inside a memory has 256 columns, so that said one image line can be stored in the Nth bank of the Nth row inside the memory. Herein, said 8 pixel data is defined as one word.
In the conventional memory access control apparatus, when the memory where the image data is stored as 32 words is accessed, a bank change is performed at each image line in order to bring the stored image data in a vertical direction of the stored image frame. In a process for sending a command for the bank change request to the SDRAM interface, a certain number of memory access latencies (for example, 6 clocks) are generated. That is, if the number of clocks necessary to access one word is 1, 32 clocks are required to bring 32 word data. Nevertheless, the number of clocks of the memory access latency generated during the bank change of 32 times is 191 (6(the number of clocks)×32 times−1 (the number of clocks)). According to this, in the conventional memory access control apparatus, a memory access efficiency of the entire system is greatly degraded, so that a high efficiency of a memory access of the entire system can not be ensured with the existing method.
As aforementioned, in the conventional memory access control apparatus, image data constituting one image frame is sequentially stored in the external memory by one-dimensional array method, so that the memory access latency for accessing image data of various display apparatuses in a vertical direction is increased.